I read somewhere (a Miss Manners book?) that the proper way to gently correct someone (and this is only if you want to make sure they know they’ve made an error so that they don’t do it again and look stupid in front of other people, not just for you to score “I’m grammatically smarter than you” points) is to repeat what they have said, except say it correctly. Example:
“I heard a nucular reactor just exploded!”
“Really? Which nuclear reactor?”
It is then their choice to either 1) acknowledge the correction out loud (“Oh, it’s pronounced “nuclear”? I didn’t realize”), 2) make a note of it for future reference without actually saying anything about it, or 3) ignore it. You have done your job.
In an email, I guess the way to go about this would be to respond and somehow manage to include both “accept” and “except” in their proper contexts. Easier said than done, maybe, but still better than being all, “Yo, you sound like a retard!” Or whatever.